1. Field of the Invention
The present invention relates to a voltage-to-current converting circuit, and more specifically to a voltage-to-current converting circuit which can be used to constitute a current driving circuit in an analog multiplying circuit operating with a low voltage and which has a balance/unbalance conversion function.
2. Description of Related Art
Referring to FIG. 1, there is shown an analog multiplying circuit having a conventional voltage-to-current converting circuit, which is disclosed in Japanese Patent Application Laid-open Publication No. JP-A-3-033989, the disclosure of which is incorporated by reference in its entirety into the present application.
In the shown multiplying circuit, the voltage-to-current converting circuit includes a constant current circuit driven by a constant current source 40 and including a current mirror circuit composed of transistors 33, 34 and 38, and a differential amplifier circuit including a pair of transistors 61 and 62 so as to convert an input voltage signal V1 to collector currents of the transistors 61 and 62, so that difference between the respective collector currents of the transistors 61 and 62 and respective collector currents of constant current source transistors 36 and 37 formed of PNP transistors, are outputted as a pair of complementary output currents I1 and I2, respectively.
The output currents I1 and I2 of the voltage-to-current converting circuit are supplied to a Gilbert cell. This Gilbert cell includes two current mirror circuits, one of which is composed of transistors 43 and 41, and the other of which is composed of transistors 44 and 42, so that the currents I1 and 12 applied to one transistors 43 and 44 of the current mirror circuits are converted into a collector current of the other transistors 41 and 42 of the current mirror circuits, respectively. The Gilbert cell also includes a first differential pair driven by the transistor 41 and composed of a pair of transistor 6 and 7 receiving a second input voltage signal V2 as a differential input, and a second differential pair driven by the transistor 42 and composed of a pair of transistor 8 and 9 also receiving the second input voltage signal V2 as a differential input,
In the above mentioned analog multiplying circuit, (1) since the voltage-to-current converting circuit does not use an emitter follower, and (2) since the transistors 41 and 42 are operated as a current sources complimentary to each other with requiring no constant current source in the Gilbert cell, an low voltage operation can be realized, and the result of the multiplication of the inputs V1 and V2 can be obtained in the form of currents I3 and I4.
Another conventional voltage-to-current converting circuit is shown in T. Tsukahara et al, "WP2.6: A 2 V 2 GHz Si-Bipolar Direct-Conversion Quadrature Modulator", ISSCC94, Feb. 16, 1994, Digest of Technical Papers, pp 40-41, the disclosure of which is incorporated by reference in its entirety into the present application. This circuit is shown in FIG. 2, in which elements similar to those shown in FIG. 1 are given the same Reference Numerals. The shown circuit is configured so that an output voltage of a differential amplifying circuit having a differential pair composed of transistors 61 and 62 is voltage-to-current converted into output currents I1 and I2 by action of emitter followers composed of transistors 36 and 37, transistors 43 and 44 connected in the form of a diode and emitter resistors 53 and 54. The shown circuit also includes a current mirror circuit composed of transistor 41 and a resistor 51, another mirror circuit composed of transistor 42 and a resistor 52, and a Gilbert cell constituted of transistors 6, 7, 8 and 9. With this arrangement, the result of the multiplication of the inputs V1 and V2 can be obtained in the form of currents I3 and I4.
In the conventional conventional voltage-to-current converting circuit shown in FIG. 1, the PNP transistors 36 and 37 are used as a load of the differential amplifying circuit for obtaining a current difference, and also in a folded path (namely, reflected current path) of the current mirror circuit. In order to realize the voltage-to-current converting circuit shown in FIG. 1, it is necessary to use a device manufacturing processor for formation of PNP transistors. Ordinarily, the device manufacturing process for an LSI including PNP transistors is more difficult than that for an LSI including only NPN transistors. Accordingly, if the voltage-to-current converting circuit shown in FIG. 1 is adopted, the cost of the LSI becomes high.
In the conventional voltage-to-current converting circuit shown in FIG. 2, since the emitter followers composed of the transistors 36 and 37 are included, it is disadvantageous in that there is required a supply voltage higher than that of a circuit having no emitter followers, by about 0.8 V of an emitter-base forward direction voltage drop. The above referred paper mentions that an operating voltage of the circuit is 2 V.